Add mypy_extensions. (#611)

Author: davidfstr

third_party/2and3/mypy_extensions.pyi

@@ -0,0 +1,6 @@
+from typing import Dict, Type, TypeVar
+
+T = TypeVar('T')
+
+
+def TypedDict(typename: str, fields: Dict[str, Type[T]]) -> Type[dict]: ...